An autosomal recessive form of juvenile Parkinsonism (ARJP) is the major cause of early-onset Parkinson's disease (PD) which results from mutations of the PARK2 (parkin) gene. Parkin is an E3 ligase that is responsible for the addition of poly-ubiquitin chains on specific substrates, which is recognized by the proteasome for degradation. Most disease-causing mutations of parkin are thought to be loss of function mutations (1), leading to the failure of parkin substrates to be ubiquitinated and degraded by the proteasome (2). Presumably, one or several of these proteins subsequently accumulate, ultimately causing neuronal degeneration in ARJP. To date, nine putative substrates degraded by parkin have been reported, two of which, p38 and Pael-R, interact with CHIP (carboxyl terminus of Hsc70-interacting protein). CHIP, like parkin, is an ubiquitin ligase, which acts in concert with parkin to modulate binding and activity of these substrates despite having separate targets (3,4). Recent work from our laboratory has shown that the levels of parkin increase in a gene dose-dependent manner in CHIP knockout mice. Moreover, the levels the parkin substrate, p38, are also elevated in these mice. Several investigators have generated parkin knockout mice in attempt to mimic ARJP in a mouse model. Unexpectedly, these mice lack frank nigral neuronal loss perhaps due to a compensatory mechanism involving CHIP. Based on the above results, we hypothesize that parkin and CHIP share common substrates such as p38, which are pathologically relevant to the disease and contribute to selective neuronal loss. We postulate that CHIP is a key factor in the regulation of parkin cell biology such that depletion of CHIP expression will result in loss of the dopaminergic neurons in the parkin knockout mice. CHIP may exert a central and unique role in PD in fine-tuning the response to proteotoxic stress at multiple levels by compensating for and regulating parkin substrates. The unifying hypothesis in this proposal is that parkin and CHIP are functionally and mechanistically linked by common effects associated with the ubiquitin proteasome system. It is our focus to resolve the mechanisms by which CHIP regulates parkin cell biology in vivo, and how this may be applicable to the clinical/neuropathological spectrum of presentation in ARJP.